1. Field of the Invention
The present invention relates to cutting techniques using a plasma arc, and in particular relates to improvements in techniques for reducing wear of the electrode or nozzle in a plasma cutting method employing a working gas containing oxygen.
2. Description of the Related Art
Plasma cutting methods using a working gas (plasma gas) containing oxygen are appropriate to the cutting of low alloy steel or low carbon steel (soft steel). It is known that, since, in this plasma cutting method, the electrode within the plasma torch is exposed to oxygen, hafnium is ideal as the material of this electrode (material of the heat resistant insert mounted at the electrode tip) (Japanese Patent Publication No. Sho. 49-8622). This is because when hafnium forms an oxide its melting point rises to about 2800-2900.degree. C., thus exhibiting excellent heat resistance. In practically all of the currently implemented plasma cutters using oxygen or air as working gas, hafnium is used for the electrode. Also, since zirconium has practically the same properties as hafnium, it is suitable for electrode material in an oxidizing atmosphere.
In plasma cutting, the nozzle and electrode are worn when the arc is generated. There are many causes of wear of the nozzle and arc, but the modes of wear that are caused by the arc itself when an oxidizing working gas is employed are as follows.
Firstly, the electrode and nozzle are worn when the pilot arc (between the electrode and nozzle) is initially ignited. At the instant when the pilot arc is ignited, the surface temperature of the hafnium constituting the electrode is instantaneously raised from room temperature to a high temperature exceeding 3000.degree. C. and at this point there occurs rapid wear of the hafnium. The wear of the hafnium in this transient condition, even though it lasts only about 0.1 second, may correspond to the wear occurring in about one minute in the subsequent condition in which the hafnium has stabilized at high temperature. Also, whilst the pilot arc (between the electrode and nozzle) exists, the nozzle is placed under the same condition as the work during cutting and so is melted away in an oxidizing atmosphere.
Secondly, the electrode is worn when cutting is performed by establishment of the main arc (between the electrode and work). Although the hafnium of the electrode is stable at high temperature during cutting, so its rate of wear is lower than when the pilot arc is ignited, wear does proceed, and the hafnium is gradually consumed like a burning candle becomes shorter.
Regarding the problem of wear of the electrode and nozzle by the arc, Laid-open Japanese Patent Application No. Sho. 61-92782 discloses a method of starting plasma cutting wherein, at the start (including not just the interval of pilot arcing, but also piercing when the main arc is subsequently established), a mixture of oxygen gas and nitrogen gas of oxygen concentration 70-10 mol % is supplied as a working gas, changeover being subsequently effected to oxygen gas. Also, Laid-open Japanese Patent Application No. H. 3-258464 (U.S. Pat. No. 487,747) discloses a method of starting plasma cutting wherein nitrogen gas is employed during pilot arcing, but subsequently a changeover to oxygen gas is effected. In this way, wear of the hafnium electrode or nozzle during pilot arcing can be reduced by lowering the oxygen concentration of the working gas or eliminating the oxygen during pilot arcing.
As described above, with the object of reducing wear of the electrode or nozzle, the oxygen concentration of the working gas is lowered, or pure nitrogen gas is employed at the start of cutting, in Laid-open Japanese Patent Application No. Sho. 61-92782 and Laid-open Japanese Patent Application No. H. 3-258464. However, in all these cases of the prior art, during cutting, pure oxygen gas is employed as working gas(Laid-open Japanese number Sho. 61-92782, Laid-open Japanese Patent Application No. H. 3-258464) or working gas of the very high oxygen concentration of more than oxygen concentration 95 mol % is employed (Japanese Patent Publication No. H. 1-9112). Consequently, wear of the hafnium electrode during cutting cannot be effectively suppressed.
Thus in the prior art, the reasons for employing pure oxygen gas or working gas of high oxygen concentration approximating to pure oxygen in plasma cutting using a hafnium electrode with an oxidative working gas are believed to be as follows.
Firstly, even if pure oxygen gas is employed (albeit there is the problem of high running costs due to the need to change the electrode frequently), the life of the hafnium electrode is practicable i.e. at least some hours, so long as particularly large-current plasmas are not employed. On the other hand, if the oxygen concentration of the working gas is lowered in order to reduce electrode wear, the cutting quality is correspondingly lowered. Lowering of cutting quality is a problem that cannot be neglected. Pure oxygen gas is therefore employed in order to obtain best cutting quality, even though it causes high running costs.
Secondly, even if a mixed gas constituted by adding for example nitrogen to oxygen gas was employed as the working gas instead of pure oxygen gas, with the conventional working gas supply system, changes occurred in the respective gas flow rates during cutting, so it was not possible to maintain the mixing ratio of the oxygen and nitrogen at the optimum value right through cutting. Specifically, in the conventional gas supply system, one or other of the following two methods was adopted in order to set the gas flow rates to the target values. The first of these is the method of setting the pressure of the gas that is supplied to the torch to the target value by means of a pressure reduction valve. Second is the method of setting the flow rate to the target value using a needle valve and flow rate meter. In both methods, the gas flow rate can only be maintained at the target value if the pressure loss within the torch is constant. However, during the cutting, deterioration of the nozzle and wear of the electrode occur, resulting in changes in the pressure loss within the torch and correspondingly causing the gas flow rate to depart from the target value. Specifically, the pressure loss within the torch decreases during the progress of cutting, resulting in a gradual increase of the gas flow rate. For example in some cases the flow rate may increase by a factor of 3 at the end of cutting compared with the new-product condition at the commencement of cutting. Thus the gas flow rate changes during cutting, so the oxygen concentration of the working gas cannot be maintained constant at the optimum value. Accordingly, simple pure oxygen gas is employed as working gas during cutting.
However, it will be most desirable if electrode wear can be reduced. In particular, in the case of an oxygen plasma with large current of about 300 A, the life of the hafnium electrode is shortened, down to about two hours; the increase in running costs and lowered working efficiency resulting from the need for frequent electrode replacement cannot therefore be neglected.